Pigeons were trained to discriminate two types of visual forms that could vary in two orthogonal dimensions. One set of stimuli was designed to have relatively integral dimensions, while the other set of stimuli was intended to have relatively separable dimensions, for the pigeon. A first experiment provided evidence that the dimensions of our two stimulus sets differed in the degree of separability. The second experiment examined the effect of increased stimulus range on discrimination of the two stimulus sets. Results from this experiment indicated that increases in range in either relevant or irrelevant dimensions reduced discrimination for the integral stimulus set. For the separable stimulus dimensions, however, performance declined only with increased stimulus range along a relevant dimension. In the third experiment, stimulus range was increased along either one or two dimensions of the stimulus. Correlated changes in stimulus range along two dimensions had an impact only for integral stimuli. In summary, these experiments demonstrate that the organizational structure of stimulus dimensions determines the impact of increases in stimulus range, probably due to the attentional properties of different stimulus forms.